It is well known that hydrocarbons fluoresce when exposed to ultraviolet illumination. An effective technique for utilizing this physical principle to detect the presence of hydrocarbons in geologic structure involves placing a long-wave ultraviolet light source ("blacklight") in a borehole to conduct in-situ fluorescence tests. Examination of the borehole wall for hydrocarbon fluorescence under ultraviolet light can indicate oil and condensates in small amounts which might not be detected by other means.
Fluorescent lamps have been found to be an efficient source of long-wave ultraviolet (UV) illumination for hydrocarbon fluorescence determination. One type of specialized fluorescent lamp emits UV light having a wavelength of 365 nanometers with only a small quantity of visible violet and blue light. This lamp, like other fluorescent lamps, requires a high initial voltage to start the lamp and a lower voltage, current-limited power supply thereafter. Previous power supplies having the necessary voltage and current characteristics for a fluorescent light system have been difficult to implement in borehole applications because of their physical size and electrical noise characteristics, as will be discussed briefly below.
Conventional fluorescent lamp power supply circuits normally provide the high initial voltage by interrupting the current in a series ballast inductor to provide a high voltage inductive spike. The ballast is designed with sufficient impedance or resistance to provide current limiting after the fluorescent tube ionizes. The ballast method is applicable to both AC and DC operation. However, because of the bulky size of the ballast, this method is very difficult to implement in borehole applications.
An alternate method for providing power to a borehole fluorescent light system employs an oscillator and transformer to provide the high voltage and current limiting features needed to operate a fluorescent bulb. Previous power supply systems based on an oscillator/transformer transformer combination are unsuitable for use in borehole television systems, however, because they tend to produce electrical noise interference which hampers the operation of video equipment. In particular, the continuous operation of a high frequency oscillator in the power supply circuit produces unwanted signals which are detected by the high gain, wide bandwidth amplifiers used to amplify the small signals from the video imaging device, such as a vidicon tube or solid state metal oxide semiconductor (MOS) imaging device.